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A Handmade Thin Section Sampling

About 30 years ago I began trying to make thin sections. I remember I was fascinated by the interference colors. I have always been an experimenter. I made a means of viewing thin sections with polarizing filters from the camera store. I bought one thin section to use as a sample in my experimenting. I still have that thin section of Cole Creek a nice H5 ordinary chondrite.

I have to tell you that my first slides were just terrible. I had some small fragments of Allende and Millbillillie that I had saved from cutting. I slivered up the fragments into slices as thin as I could and I smoothed the tiny little bits on one side and glued them down to glass slides. The only slides I had on hand were biological size slides and their glass was thinner than petrographic slides. I did not have a way to hold the slides and use a lapping machine so I sanded the meteorite bits down with 400, 600, and 1000 wet/dry aluminum oxide sandpaper. Wow was that tough to do. And as I said the results were awful but I managed to see interference colors enough to press on for better methods. I did a little reading about making thin sections. There was no internet and information was scarce. I made a few more and they were better and the meteorite pieces were getting bigger. I started sanding the slides carefully on the lapping discs pretty soon.

The Meteorite Exchange had one meteorite and later two more meteorites needing to be classified and I had to cut off a piece of the three to send to labs. I was sort of eager to get a few thin sections of each to play with. Paul Harris and I decided that we would get a few thin sections made and offer them for sale along with the slices and pieces of the meteorites after they were officially classified. So I sent a prepared chunk of each meteorite off to a thin section manufacturer and told them to make ten slides of each meteorite if there was enough material to do so. My instructions were to make half polished uncovered and half covered. Back then it was not expensive to have this done and it did not take very long. That is a pair of things that have changed a great deal over the last twenty-plus years. Now it costs a lot and takes forever. We got the slides back and I had more samples to play with for a while to refine my viewer machines and get a better knowledge of how different meteorites appeared in finished slides. The first meteorite we did this with was Sahara 99676 we found that the slides did sell and we were not stuck with leftovers. It took a long time to get the others classified and we held the slides till they were official and then found again that a few slides would sell quite fast. But that was the last time we offered slides along with meteorites.

I took the opportunity to examine thin sections whenever I saw them at the Tucson Gem show. I was getting myself ready to try again at making my own. I knew enough now to be dangerous as they say. But more importantly, I had a plan this time and ideas about tools to give me some control as I ground the slides. Half the fun for me is figuring out how to do things no one else would ever try to do for themselves. I had bought my first box of petrographic slides months before and had manufactured some fixtures to hold the slides that I could hold as I worked the meteorite material down through the different lapping disc grits. My first attempts were the two meteorites that we continued to wait to become official. My goal was nearly full surface coverage and evenly thin slides that looked the same as the professionally made ones. I still had those on hand unsold. Not having equipment designed for the job meant my workflow was different from a commercial maker. I cut all the thin slices first and then ground and polished one side instead of cutting and polishing off a prepared chunk. The finished results before grinding were the same. Flatness on the first polished side was never an issue for me. The polished side of the slices was glued to the glass slides and mounted to my fixtures. I ground down at 150 grit until the meteorite was getting just transparent in spots. I worked in stages through different grits as the material got thinner and thinner. I went really slow in the beginning years and my finest lapping disc was 600 I think back then. I got a 1200 grit disc eventually and that gave me more control near the end of grinding and I could go straight to polishing without having to hand sand the 600 down to 1000 on sandpaper before polishing on plastic polishing film.

The process described above has changed only a little over time. I now final polish the thin sections on a second lapping machine which is a dedicated polisher. Also, I now have 1500 and 3000 grit lapping discs, and all lapping discs are 8 inches instead of 6 inches. The process surprisingly has gone from several hours to make a single thin section with fear and trepidation to about one hour with nicer results. Most of the thin sections now are close to that 30-micron magic thinness. I admit the ones at first were often not thin enough. However, I have gone back and thinned most of those down. My viewing equipment is still the same modified microfiche machine and my Canon DSLR mounted on an extreme close-up rig.

Microscopes often have a reticle eyepiece that will allow the user to measure the objects being viewed. I have nothing so fancy in my thin section photography. The gap between the end of the camera and the top of the slice is never more than one quarter of an inch and often far less. So with difficulty I can shine a light into the gap with a machinist rule in place of the thin section and take a direct photograph of my field of view using the 1/100th of an inch scale. The images for this article were taken with the exception of the last image with a field of view of just .33 inches (8.382 mm) wide. The final image of the Lunar meteorite with the inclusion had a field of view just .17 inches (4.318 mm) in width.

Field of View image showing 33/100th of an inch

So that’s the story of my thin section collection. I have only four thin sections that I have bought. I have made over a hundred and fifty from meteorites in my collection or from ones I have cut for the business or the few friends I do cutting for. Sometimes I take my pay for cutting as a slice or two and if I can I make one slice very thin for a thin section or two. The following images are all from thin sections I hand made. A few are too thick I admit. But there are occasions where it is never going to be possible to obtain any more material so I have stopped just a little short rather than grind too much. I can always take off more later but I can never put any back. They are all polished and uncovered thin sections.About three years ago my methods and tools took another jump forward along with more care from me. One of my slides was used to classify a meteorite. The classifier told me the professional thin section maker was taking forever to do the work and the thin sections were often too thick. I offered to prepare a section for the meteorite I was submitting. They agreed and they used it. I made seven thin sections of that stone (NWA10991) and they were all very nice and correctly thin. My slide did have to be repolished for the microprobe work since I use diamond to polish everything now. They needed to remove any diamond residue and used colloidal silica to repolish it.

These is the batch of thin sections selected for this article

Two of the meteorites we sold professional thin sections of were NWA 774 and NWA 775. My homemade copies of those were used for the images below.

NWA 774 is an H4 ordinary chondrite.
NWA 775 is an L6 ordinary chondrite.
NWA869 L3-6 Chondrite

NWA869 was one of the last of the experimental thin sections before I had any tools to support the slide as I ground it down. It was crude and uneven. Yet it gave me a bigger surface and more interference colors, especially around the edges. I have repaired the poor work since that original experimenting and it is not a perfect thin section but is much better, thinner, and flatter. The image above is from the repaired slide.

NWA8008 L4 ordinary chondrite

NWA8008 was sent off to be classified with the hope that it would be a type 3 ordinary chondrite. I made three thin sections from slivers I cut off the meteorite. I was unsure if it would get the type 3 classification but was happy with the thin sections I made. They were getting thinner, the surface area was getting near to fully covering the glass slide. It was classified as a type L4. This was the second phase of my learning to make thin sections. I was happy with the results.

Quitovac L5 ordinary chondrite

Quitovac was a meteorite that a friend of mine, Jason Phillips acquired directly from the finder. He asked me to clean the masses and do some cutting on them. I got a piece of the stone for my collection and made four thin sections from it. I gave Jason one. They were nice enough that I was not at all ashamed of the work. That point was about 4-5 years ago. They were still made with the 1200 grit lap as the finest I could go and still hand polished. They were close to the correct 30-microns. I have looked at them several times and decided they can not be improved by further work. I am still using the way they appear at high magnification to judge when they are finished. I have gone too far a few times grinding them. They become less colorful than they should be. It is now more art and feel than science.

Dhofar 1289
Al Haggounia 001
Al Haggounia 001
R Chondrite
R Chondrite

I made up a batch of prepared slides before we moved. I did not have time to work on them until two years after being in the new house. It was four slides each of three meteorites and one slice of an R3 chondrite that was from a cutting job. I have never gotten the official number on the R chondrite so it is just labeled “R” on the thin section. The other two meteorites were Dhofar1289 and Al Haggounia 001 which are pictured above. Dhofar1289 is another L4 and a very nice stone in a thin section. I chose to image chondrules in Al Haggounia 001 since it was for so long officially an Aubrite despite many people knowing it had chondrules. It is now reclassified as a “vesicular, incompletely melted, EL chondrite impact melt rock.”

I love type 3 chondrites and long-time readers of this article know that well. I finally had a series of meteorites that I submitted come back as type 3s. I made thin sections of each to view and enjoy as I waited for the official results. The following images are of one of those meteorites.

NWA11991 LL3 Ordinary Chondrite
NWA11991 LL3 Ordinary Chondrite

NWA11991 is an LL3 and a stone I never get tired of looking at under polarized light. It shows beautiful crisp chondrules. Also, some are tightly packed and squeezed, even deformed into a clustered chondrule texture in areas. This was classified using a brother or sister slide of the one imaged here.

NWA10816
NWA10816

NWA10816 is an LL5 and a beautiful meteorite. The thin sections were getting pretty easy to make by the time I did this meteorite. I usually do several. It is always cool to see the things in another thin section that would be missed if I just had a single slide.

I did a batch just a few days ago. Included in the group were two slides for Rick Kujawa. He had bought a little stone that was supposed to be a type 3 chondrite from the pictures he was sent. I received the stone in the mail. Rick asked me to cut off the end and then two thin slices to make into a thin section for each of us. I did the cutting and polished up the endpiece and face of the main mass. Then prepared the two thin slices to be made into thin sections. It all went well on the slide that was going to be mine until right at the end. A crystal grain pucked loose and took out a part of the thin section as it passed between the slide and the 3000 lapping disc. My slide became Rick’s slide and I finished them both off just a little too thick for laboratory work. I got chicken about going too far. I could not remove any more slices from his small meteorite. I think from the image below that he does have a type 3 chondrite.

Rick’s Unclassified

Several years ago, I was given as a gift a stone from Erich Haiderer. I took it home and cut an endpiece off. Of course, I made a thin slice to get some thin sections from too. The images following are from those thin sections. It is a nice meteorite. Clearly, it is not a type 3. The chondrules are blasted, and mineral grains are displaced from the rest of the chondrule. Still, it has a lot going on in it as can be seen below.

Erich Haiderer Unclassified Chondrite
Erich Haiderer Unclassified Chondrite

Just one more ordinary chondrite to present here. It is a meteorite that is still under classification at the moment. It is a pretty thing too. I have confidence that it will be officially another type 3. As the following images display it is a mass of crisp chondrules border to border.

Chondrite Under Classification
Chondrite Under Classification

I have trouble with fresh Carbonaceous Chondrites. I have to develop a method so that they come out well. I may have started with tiny bits of Allende decades ago in this journey, but I still can not make a big one look right yet. But older C chondrites do ok. Here are two images of NWA7678 a CV3.

NWA7678 CV3 Carbonaceous Chondrite
NWA7678 CV3 Carbonaceous Chondrite

Impact melts meteorites are just a bit more difficult for me to do. The work is not harder, or different but they must be the right thinness. I think it is the melted rock that is just more opaque until the very end of grinding that is the problem. The melt portion sometimes seems to remain dark. Caldwell is an L impact melt breccia. I had a small piece. It was a broken sliver with nothing going for it appearance-wise. Fortunately, both the chondritic and melt lithologies were in the sliver. It made a nice thin section once I got it thin enough. The melt side image shows an almost achondrite appearance with what appears to be spindle-like crystals. The chondritic portion of the thin section has blasted and destroyed chondrules but retains its chondritic texture like an L6 might have.

Caldwell melt portion
Caldwell chondritic portion

I got NWA7347 at the Tucson Gem and Mineral show. It was weird, funny looking, and the inside was a very odd slate gray color. I have written about this before. I made just a single thin section of this meteorite. It turned out to be a wonderful L6 melt breccia. As with Caldwell, the slide shows both the chondritic and the melt portions of the stone. The melt image is the lighter color in the meteorite when seen in a thin section. The numerous black ovals seen in the “melt” image are tiny blebs of melted nickel-iron that form a silvery sheen when the meteorite is viewed with the unaided eye.

NWA7347 melt portion
NWA7347 chondritic portion

Millbillillie was one of my first experiments. It was a terrible slide, but I was able to see the wonderful crystal texture that eucrites have. I was cutting a new eucrite for a friend about two years ago and got a piece to make two thin sections. There was that same texture but in a much larger area of the glass, thinner and more even. I left the thin section just a little thick hoping that I will further refine my process. Then I can remove that tiny bit of thickness without losing any of the intertwining crystal structure. Here are two images of that Eucrite. You can see that it could be just a bit thinner.

Eucrite
Eucrite

The newest classes added to my homemade thin section collection are Lunars and Martians. And Lunars have to be close to that 30-micron thinness to show their stuff. They end up little more than a faint gray smudge on the glass. The Martian thin sections of NWA12269 are from a couple of years ago. They may need to be retouched a bit. They have areas of darkness, but I did not want to rush to do the work because I have read a little and seen professional thin sections that also have those dark areas. The large dark areas of blue gray are not bad spots in the slide they are numerous crystal grains of an isotropic mineral that shows no interfence colors and acts like glass between the polarizers. The spidery white lines are the edges of the crystals. Many of which are quite shattered into tiny bits.

NWA12269 Martian Shergottite
NWA12269 Martian Shergottite

The first lunar image is from NWA11303. Lunar thin sections are just neat in every way. They are from The Moon, and I get to see everything that is in them.

NWA11303 Lunar meteorite (feldspathic breccia)

I am including a single image from another Lunar meteorite at the end. There is a bit of what I want to think is chondrite material. It looks like barred olivine seen in many chondrite stones. However, I am just an amateur. I can not say for sure that it came from a meteorite that hit The Moon and became part of the breccia of NWA11474. But I like to think so.

NWA11474 Lunar meteorite (feldspathic breccia)

Making thin sections by hand is not easy. It requires patience and care. I have enjoyed the learning process and still have not gotten professional equipment to make them. To do that would take the hobby out of it for me. It has been a nice sideline part of my meteorite collecting.

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